1. Field of the Invention
The present invention generally relates to a mobile communication system. More particularly, the present invention relates to an apparatus and method for establishing a call through random access between a Mobile Station (MS) and a Base Station (BS) in a mobile communication system.
2. Description of the Related Art
Today's mobile communication systems are evolving to high-speed, high-quality wireless data packet communication systems that additionally provide data service and multimedia service beyond early-stage wireless voice services. Universal Mobile Telecommunication System (UMTS), a 3rd Generation (3G) mobile communication system that operates in Wideband Code Division Multiple Access (WCDMA) based on European mobile communication systems, Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), provides uniform services that enable mobile phone users or computer users to transmit packet-based text, digitized audio or video, and multimedia data at or above 2 Mbps all around the world. The UMTS system adopts the access concept of packet switching using a packet protocol such as Internet Protocol (IP) and enables access to any end within a network at any time.
The 3rd Generation Partnership Project (3GPP) working on standardization of UMTS is discussing Long Term Evolution (LTE) as the future generation of UMTS. LTE realizes packet communications at high rates, around 100 Mbps. For this purpose, many techniques have been discussed, including reduction of the number of nodes on a communication line by simplifying a network configuration and optimizing wireless protocols to radio channels.
A Radio Resource Control (RRC) state, which has been defined to distinguish User Equipment (UE) operation modes and communication states, includes an idle mode and a connected mode. A UE in the idle mode or connected mode performs an Random Access (RA) procedure in order to acquire synchronization according to UpLink (UL) timing information received from an Enhanced Node B (ENB) of an Enhanced UMTS Terrestrial Radio Access Network (E-UTRAN), to perform power control setting for initial UL transmission, or to transmit an RRC message to the ENB.
FIG. 1 illustrates a signal flow for an RA procedure in a conventional 3GPP E-UTRAN system.
Referring to FIG. 1, a UE 110 starts the RA procedure for a new call connection in idle mode. The UE 110 selects one of the available RA codes based on system information received from the ENB 111 in step 121 and transmits an RA Preamble based on the selected RA code to the ENB 111 in step 131. A channel for delivering the RA Preamble and a transmission time of the RA Preamble are indicated by the received system information.
The ENB 111 receives the RA Preamble, calculates a UL timing with the UE 110, and transmits an RA Response including timing synchronization information to the UE 110 so that the UE 110 can acquire UL timing synchronization in step 132. The RA Response includes code information that the ENB 111 has received in the RA Preamble and resource allocation information by which the UE 110 can perform scheduled transmission.
If the code information included in the RA Response is identical to the RA code transmitted in the RA Preamble, the UE 110 transmits a scheduled transmission message according to the resource information included in the RA Response in step 141. The scheduled transmission message is a Layer 3 RRC message, for example. In step 142, the ENB 111 transmits an RRC message to the UE 110 to establish a signaling radio bearer for exchanging Layer 3 control messages with the UE 110. If a UE Identifier (ID) included in the RRC message does not identify the UE 110, the UE 110 re-starts the RA procedure.
As described above, since the 3GPP E-UTRAN uses only an RA code in the RA procedure, the ENB has no knowledge of information about the call requested by the UE. As a result, when a preamble contention occurs among a plurality of UEs during RA, the setup of a call requiring fast call setup (e.g. an emergency call with a high priority level) is delayed.